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@drHannibalLecter, I should've used some examples in English, like 'sell' in 'The book sells good' is an active form with passive meaning. :)
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NS.X.Aug 12 '12 at 19:00

Well, that makes sense now. Is there a fixed number of Chinese verbs which can have this form with 被, or you can "convert" any verb that fits the sentence?
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dr Hannibal LecterAug 12 '12 at 19:19

1

@NS.X. I prefer to think of this as an example of an ergative verb. en.wiktionary.org/wiki/ergative_verb There are many ergative verbs in Chinese, just like in English. Another example would be 打敗 (to defeat someone, or to lose).
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deutschZuidAug 12 '12 at 21:47

It is OK to remove the "被”; we use both versions in daily life. If you add "被", you emphasize the passive form, and in this sentence, this kind of "passive form" can be omitted because the meaning of this sentence will not change if you omit the "被".

I think this is different from English. English is used to emphasize the relationship of object by grammar, but Chinese is used to emphasize relation by meaning. And in this sentence "病" was cured, it is "passive form", even you don't add "被", everyone knows "病是被动的", so you can omit it.

If you add "被", it is very very correct, but if you do not add "被"，no one can say that is wrong.